Chronic infection with hepatitis B virus (HBV) is an important risk factor in the development of hepatocellular carcinoma (HCC). We present a model for the role of HBV in HCC, derived from our demonstration that the HBV transactivator X protein (HBx) interacts with XAP-1/UV-DDB, a cellular protein involved in nucleotide excision repair (NER), and that HBx expression inhibits normal NER. We hypothesize that HBx binds XAP-1 to provide a function to the HBV life cycle, and that the protein interaction cripples DNA repair and inadvertently contributes to the development of HCC. We will test the model utilizing several approaches and the following specific aims: (1) Effect of HBx on cellular DNA repair. The domains of HBx responsible for inhibition of NER will be mapped using mutant HBx proteins. Fibroblasts from Xeroderma pigmentosum (XP) patients will be used to determine whether HBx inhibits DNA synthesis through a pathway involving XAP-1. The effect of HBx on mutation frequency will be tested utilizing our HBV X transgenic mice. The possibility that HBx interference of DNA repair requires alteration of a p53-mediated pathway will be examined. (2) Investigation of functional interactions between cellular XAP-1 and HBx proteins in the liver using a mouse model. Murine XAP-1 will be cloned. Cellular DNA repair assays will measure functional effects of HBx-XAP-1 protein interactions within the livers of HBV X transgenic mice. The effect of HBx expression during liver regeneration will be tested. (3) Determination of a role for DNA repair protein XAP-1 in the HBV life cycle. A role for XAP-1 in the repair of the HBV partial double-stranded DNA genome will be tested. A panel of HBx mutant proteins will be used to investigate the requirement for XAP-1 in HBx transactivation. The proposed program builds on our model of HBV-related HCC and suggests a novel mechanism of action for indirect-acting human tumor viruses such as HBV. The experiments will provide new insights into HBV effects on hepatocytes, viral replication, and the development of HCC.